Electrical deposition of material

ABSTRACT

Insulating particles are deposited on an insulating support bearing a conductive film by positioning it between an anode and cathode in an electrically conducting solution so that it faces the anode, but without electrical connection with the anode or cathode, connecting the conductive film to a conductive part in sight of the cathode and establishing a potential difference between the cathode and anode.

United States Patent Electric & Musical Industries Limited [73] Assignee Hayes, Middlesex, England [32] Priority Nov. 8, 1967 [33] Great Britain [31 50,821/67 [54] ELECTRICAL DEPOSITION 0F MATERIAL 7 Claims, 1 Drawing Fig.

[52] U.S.Cl. 204/l8l [51] lnt.Cl B01k5/00 [50] Field oi Search 204/181 [56] References Cited UNITED STATES PATENTS 1,590,596 6/1926 Taylor et al 204/181 X 1,641,322 9/1927 Czapek et al. 204/ l 8] X 2,898,279 8/ l 959 Metcalfet al 204/l8l Primary Examiner-Daniel E. Wyman Assistant ExaminerW. Cannon Attorney-William W. Downing, Jr.

ELECTRICAL DEPOSITION F MATERIAL This invention relates to the electrical deposition of insulating material and in particular to the deposition of a thin layer of particles onan insulating support.

It is known to deposit insulating particules on an insulating support by an electrophoretic process. To carry out this process the insulating support is provided with an electrically conductive film and is immersed in an electrically conducting solution in which the insulating particles are suspended, a second electrode is provided and this electrode and the elec trically conductive film are connected as anode and cathode respectively to the terminals of a source of voltage. This process sufiers from the limitation that the conductive film has to be relatively thick to carry the full current which passes through the solution.

One object of the present invention is to provide an improved method of depositing a thin layer of particles on an insulating support, with a view to reducing the limitation referred to in the preceding paragraph.

According to the present invention there is provided a method of depositing a thin layer of insulating particles on an insulating support which comprises the steps of providing a surface of the support with an electrically conductive film, positioning the support in an electrically conducting solution in which said particles are suspended so that the support is located between the two electrodes in said solution, and, without electrical connection being made to said conductive film, establishing a potential difference between said electrodes to cause said particles to be deposited on said conductive film.

By virtue of the invention, a relatively large current can be passed between the two electrodes, but the conductive film takes only the current needed to prevent the build up of electrostatic charges on the insulating support. Therefore, although the conductive film acts as an electrode in addition to the other two electrodes it need only be of the order of a few tens of angstroms thick. The invention is thus especially suited to processes where it is desired to deposit insulating particles on a thin conducting film without damaging the film. Alternatively, should it be desired to remove the film after deposition of the particles, this is facilitated due to the thickness of the film.

The conductive film is preferably antimony and it can be substantially dispersed by washing in running water or by baking in air at about 400 C. or both.

It is found advantageous to connect the conductive film electrically to a conductive ring which acts as a frame for the insulating support and which is in sight of both the other electrodes. The charge deposited with the particles thus spreads through the ,conducting film and passes back into the solution from the portion of the ring which is in sight of the cathode, to which it then travels.

ln order that the present invention may be clearly understood and readily carried into efiect it will now be described with reference to the single FIG. of the accompanying drawing which shows in diagrammatic form one stage in the method of producing a luminescent screen for an image intensifier.

Particles "of the phosphor to be deposited as the screen are put into suspension by adding 780 milligrams of the phosphor to 900 milliliters of isopropyl distilled alcohol. The phosphor in this example is a fine grade zinc silver sulfide (ZnSAg). The suspension is subjected to ultrasonic agitation for about minutes. During this period 21 milliliters of zirconium nitrate N/IOO solution are added. On completion of the ultrasonic agitation the suspension is ready for use and it is put into a transport cell 1 which is illustrated diagrammatically in the drawing.

The insulating support 2 on which he phosphor particles are to be deposited is in the form of a fine glass or mica sheet mounted on an electrically conducting ring 3. Before being placed in the cell 1, the support is provided, on that surface on which the particles are to be deposited with a conductive film 4 of antimony which extends over the edge of the ring 3 or is otherwise electrically connected to it. The film of antimony is applied by evaporation of 20 milligrams of antimony from a tungsten spiral placed about 6 inches from the support. The antimony film is only a few tens of angstroms thick and it causes a reduction of the light transmission of the support only by about l5 percent. 7

After the support has been provided with the conductive film of antimony, it is immersed in the suspension 7 in the transport cell 1, as indicated in the drawing, between an anode 5 and a cathode 6. The electrodes 5 and 6 are about 4 to 5 centimeters apart and the insulating support 2 is about 3 to 4 centimeters from the anode 5 and is positioned so that the conductive film 4 faces the anode. Then, without making electrical connection to the ring 3 or the film 4, the anode and the cathode are connected to the respective terminals of a suitable source of potential, of say between 1000 and 200 volts. The potential is arranged to produce a current through the liquid suspension 7 of about 14 m. With a current of this magnitude, a monolayer 8 of phosphor particles (that is a layer of single particle thickness) is deposited on the conductive film 4 in about 7 minutes.

After a monolayer of phosphor particles has been formed on the conductive film 4 no further particles are deposited on areas where particles have already settled because the phosphor material is virtually an insulator on which no further deposition occurs. Prolongation of the settling time beyond that required to produce the monolayer serves only to ensure full coverage of the conductive film 4 by phosphor material. As the conductive film 4 takes only the current needed to prevent the buildup of electrostatic charges on the support 2, uniform continuity of the film over the surface of the support 2 is not so essential as if the film were used as the cathode 6, and minor discontinuities have less tendency to cause uneven distribution of the phosphor particles.

After the deposition of the phosphor layer on the conductive film 4, the support 2 is removed from the transport cell and the support is rinsed for approximately 7 minutes in running demineralized water. This removes most of the antimony film. Subsequently the luminescent screen so formed is aluminized, and thereafter the nitrocellulose film deposited in known manner on the phosphor particles as a preliminary to aluminizing is removed by baking the screen at 400 C. in air about l0 minutes. This baking also serves to oxidize and/or remove residual antimony from beneath the phosphor particles, so that the final efiect of the antimony on the transparency of the support is slight. It will therefore be appreciated that the removal of rendering innocuous of the an timony film is achieved by steps which would be taken even if the phosphor layer has been settled by precipitation not utilizing the conductive film 4.

The suspension in the cell 1 may be used for settling a number of luminescent screens but it is preferable to subject the suspension to ultrasonic agitation for approximately 30 seconds before each subsequent settling process.

As the layer of phosphor particles is a monolayer, screens of very fine texture are obtained, capable of producing images of high definition. After removal of the antimony the particles remain on the insulating support 2 as tightly packed adherent layer which can be removed only by relatively vigorous mechanical scraping.

If the conductive film 4 is not subject to chemical attack during the settling process, extended settling periods or high currents, of about 50 m, can be used. The use of substantial electrodes 5 and 6 enables such high currents to be handled, and as a result the invention can be used for the deposition of phosphor particles of larger size, which would normally drop out of suspension in the time required for electrophoretic deposition using smaller currents.

The invention is not confined to the formation of luminescent screens such as used in image intensifiers, and it can be used for the formation of other screens and indeed for the deposition of insulating materials other than phosphor particles, for example, particles of magnetic material such as gamma ferric oxide. Moreover the conductive film provided on the insulating support need not of antimony, and aluminum can be used as an alternative in the example described above. As the conductive film can be very thin, producing substantially little light attenuation its removal is not absolutely necessary, and any residue which is left after the normal processes of washing and baking can be allowed to remain.

If desired, a stainless steel stocking-stitch mesh may be inserted between the anode and the support 2 during the settling process, to counteract the turbulence in the vicinity of the electrode 5. Other modifications may also be used in the process described.

For example, instead of connecting the conductive film 4 to a ring such as 3 which is in sight of both the anode 5 and the cathode 6, an annulus of conductive material connected to the film 4 may be provided on the surface of the support 2 which faces the cathode 6.

What we claim is:

1. A method of depositing a layer of insulating particles on an insulating support which comprises the steps of providing a surface of the support with an electrically conductive film, positioning the support in an electrically conducting solution in which said particles are suspended so that the support is located between two electrodes in said solution, and, without electrical connection being made to said conductive film, establishinga potential difference between said electrodes to cause said particles to be deposited on said conductive film, in which the support is positioned in said solution by mounting on an electrically conductive ring which is electrically connected to the film and in sight of both said electrodes but which is not electrically connected to said electrodes.

2. A method according to claim 1 in which said conductive film is formed of antimony.

3. A method according to claim 1 in which said surface is washed alter deposition of the particles thereon so that the conductive film is at least partially removed.

4. A method according to claim 1 in which the support is baked in air afler deposition of the particles to oxidize and/or remove the conductive film or the residue thereof.

5. A method according to claim 4 in which the baking is car ried out after washing.

6. A method according to claim 1 in which said particles are phosphor particles.

7. A method of depositing a layer of insulating particles on an insulating support comprising the steps of providing a surface of the support with a thin electrically conductive film electrically connected to a further electrically conductive part which can be seen from the side of said support opposite said surface, positioning said support between an anode electrode and a cathode electrode in an electrically conducting solution containing said particles in suspension so that said conductive film faces anode and said further conductive part faces said cathode, and, without electrical connection being made to said conductive film or said further conductive part, establishing a potential difference between said two electrodes to cause said particles to be deposited on said conductive film.

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2. A method according to claim 1 in which said conductive film is formed of antimony.
 3. A method according to claim 1 in which said surface is washed after deposition of the particles thereon so that the conductive film is at least partially removed.
 4. A method according to claim 1 in which the support is baked in air after deposition of the particles to oxidize and/or remove the conductive film or the residue thereof.
 5. A method according to claim 4 in which the baking is carried out after washing.
 6. A method according to claim 1 in which said particles are phosphor particles.
 7. A method of depositing a layer of insulating particles on an insulating support comprising the steps of providing a surface of the suppOrt with a thin electrically conductive film electrically connected to a further electrically conductive part which can be seen from the side of said support opposite said surface, positioning said support between an anode electrode and a cathode electrode in an electrically conducting solution containing said particles in suspension so that said conductive film faces anode and said further conductive part faces said cathode, and, without electrical connection being made to said conductive film or said further conductive part, establishing a potential difference between said two electrodes to cause said particles to be deposited on said conductive film. 